| Chem 431 |
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Fall 2007 |
| Lecture Notes: 10 October |
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| PREVIOUS |
CARBOHYDRATES, cont.
DISACCHARIDES, cont.
Last time looked at maltose and lactose.
The first two are reducing sugars, that is they have "free" aldehyde groups, whereas sucrose and trehalose have both carbonyl groups tied up in the relatively stable glycosidic bond. Maltose and fructose are joined in alpha-glycosidic bonds. In general the alpha-glycosidic bond is easily cleaved (it is less stable chemically and organisms have enzymes to cleave it) whereas the beta-glycosidic bond is very difficult to break down.
An exception for mammals is the ability of nursing animals to digest lactose, for which the special enzyme Lactase is provided. Note that this ability is generaly lost at the age of weaning, at which time the animal becomes lactose intolerant.
Sucrose, Sucrase (Invertase), and the magic of liquid filled chocolate covered cherries.
Can have both homo- and heteropolysaccharides. We will focus on homopolysaccharides as most central, but will mention some heteropolysaccharides to illustrate their functions. Homopolysaccharides have a single type of residue. Most common polysaccharides contain glucose. Used for energy (food) storage (starches and glycogen) and structure (cellulose).
Starch (energy storage in plants). Two kinds
500,000. (text Figure 7-15b,c) Broken down by alpha-amylase (pancreas and salivary glands; random cleavage of alpha-1,4 links) to give glucose and maltose; or beta-amylase (plants; hydrolyses from reducing ends to give maltose). When either of these enzymes attack amylopectin they are blocked when they reach or are near a branch, thus end up with a "limit dextrin."
Glycogen: animal starch. Just like amylopectin, but more highly branched (every 8-12 residues). This allows more free ends for more rapid breakdown-important in animals.
Cellulose: beta-1,4 linkages, thus resistant to breakdown (including acid hydrolysis) as want for structure (don't want to digest self). Multiple, extended strands come together as fibrils held together with H-bonds (text Figure 7-16a,b), laid down in cell wall in criss-cross pattern, glued together with polyalcohols (lignin).
Remember, the beta-glycosidic bond is very difficult to break down. Thus cellulose which is linked by beta-bonds, can essentially only be digested by bacteria because of this bond.
So animals can't digest cellulose! You may ask, What about Cows and things? Well they use bacteria. Cows for instance are basically walking fermentation tanks.
Cool biological examples of cellulose use by animals: Desert Iguana consume feces to maintain culture; Rabbits eat and reprocess first pass feces (soft) to take advantage of fermentation; Multiple stomachs in Ruminants; Ultimate symbiosis in some termites: protozoans in gut have bacteria in gut, and use spirochetes as "cilia" (rowers).
 Pathway Diagrams |
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Last modified 12 October 2007
